Saturday, September 06, 2014

Down Day Yesterday - Now Norbert And ??

First - yesterday was a very suppressed, down day for most of Arizona. This is the typical situation following a day of widespread storms and rain. Above shows CG flashes for 24-hours ending at 2 am MST this morning - compare to similar chart posted yesterday.

Norbert is progressing slowly northward and is off Baja coast. The outer edges of Norbert's cloud shield extend northward and eastward across southern Arizona. Norbert is now a category 3 hurricane according to NHC, with maximum winds of 120 mph. The forecast situation next several days will be very complex. First issue will be sunshine and day time heating - clouds are currently very extensive.

The next issue will be low-level winds over southeast Arizona. There is a cold front over eastern and southern New Mexico and it is forecast by the models to push into Arizona today. The early versions of the WRF model both forecast the east winds, but both also tend to keep a nice convergence zone along the leading edge of easterlies - this tends to wobble back and forth some but stays near the metro area - one of those situations where storm development locations are critical wrt to what happens in the city.

The forecast wind profiles are very good for organized outflows with easterly steering flow and and westerly winds aloft at anvil level.

So there are many pros and cons at play with this event - starting of course this afternoon. Tropical storms have been associated with some of the biggest rain events and floods in southeast Arizon, but some have also led to much hype and anticipation but not much weather. Some of us remember Nora a few years back - she ended up being nick-named "NO Rain At all."

The two versions of the early WRF model predict quite different outcomes today for Pima County. The NAM version above forecasts limited shower activity and mostly light rain amounts across all of southern Arizona (above is forecast of total rainfall through 5 am MST tomorrow morning).

The GFS version (same forecast from GFS below) forecasts much stronger storms, much greater coverage, and heavier rainfalls. This seems to occur for two reasons - the GFS version develops CAPE this afternoon that is almost twice as great as that forecast by WRF version, and the strong storms produce organized outflows along which organized storms propagate. 

Note that neither version forecasts much in the downslope region to our east which limits potential for storms organizing before moving into eastern Pima County.

So as usual - Nothing is Easy.

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